Conventionally, there has been proposed an industrial robot that includes a plurality of links rotatably connected to each other and capable of performing a predetermined operation with the use of an end effector (arc welding torch or the like) attached to the distal end of an arm of the robot while driving the link in a rotatable manner by using an actuator (see Japanese Patent No. 4529456, for example).
In the above-mentioned robot, the actuator and the link are connected to each other by way of a power transmission mechanism including a gear or the like, and the driving force of the actuator is transmitted to the link by way of the power transmission mechanism. Furthermore, in the above-mentioned robot, a scissors gear is used as a gear to eliminate backlash thereby. Here, the scissors gear is, for example, constituted of two gears stacked on each other in place of one gear generally used and a spring that applies a biasing force to the gears in the rotational directions different from each other.
However, in the above-mentioned robot, there has been the possibility that the increase in size of the scissors gear causes the increase of the occupation space of the power transmission mechanism. To be more specific, in the scissors gear in the above-mentioned robot, housing grooves are respectively formed in the overlapping surfaces of the gears overlapping each other, and a spring is housed in the housing grooves.
That is, in the scissors gear, the spring and the housing grooves are arranged in a space between the bottom of a tooth and the center of a rotational axis of each gear; that is, at a position spaced apart from the bottom of the tooth of each gear in the direction perpendicular to the rotational axis of each gear (in the diameter direction). Therefore, there has been the possibility that the above-mentioned scissors gear is increased in size by an amount of the space for the spring and the housing groove and the occupation space of the power transmission mechanism is increased.
Actually, the larger the occupation space of the power transmission mechanism is, the larger the robot as a whole becomes. As a result, for example, when the robot performs a predetermined work, there gives rise to a possibility that the robot interferes with an object to be worked (workpiece). Accordingly, it has been desired to miniaturize the scissors gear to reduce the occupation space of the power transmission mechanism including the scissors gear.